1. Field of the Invention
The present invention relates to a transmission circuit to be used in radio communication devices such as mobile phones and wireless LAN devices.
2. Description of the Background Art
Conventionally, one of the known modulation methods used for a transmission circuit of a radio communication device is one in which an input signal is separated into an amplitude component signal and a phase component signal, and modulation is performed based on these signals to generate a transmission signal. An example of such a known modulation method is a polar modulation method disclosed by Non Patent Document 1.
FIG. 13 is a block diagram showing a configuration example of a conventional transmission circuit 1201 to which the polar modulation method is applied. The transmission circuit 1201 includes input terminals 1202 and 1203, a coordinate system converter 1204, LPFs (low-pass filters) 1205 and 1206, an angle modulator 1207, an amplitude modulator 1208, and an output terminal 1209.
An I signal (an in-phase signal) and a Q signal (a quadrature-phase signal) are inputted into the coordinate system converter 1204 via the input terminal 1202 and the input terminal 1203, respectively. The coordinate system converter 1204 generates an amplitude signal and a phase signal based on the I signal and the Q signal, respectively. The amplitude signal is inputted into the amplitude modulator 1208 after being band-limited by the LPF 1205. Also, the phase signal is inputted into the angle modulator 1207 after being band-limited by the LPF 1206. The angle modulator 1207 performs angle modulation based on the phase signal, thereby generating an angle-modulated signal. The angle-modulated signal is inputted into the amplitude modulator 1208. Based on the inputted amplitude signal, the amplitude modulator 1208 performs amplitude modulation on the angle-modulated signal to generate a transmission signal, and outputs the transmission signal via the output terminal 1209.